This invention relates to a fuel injection valve for internal combustion engines, in which the nozzle holes have its total discharge area variable in response to operating conditions of the engine.
In a conventional fuel injection valve employed in Diesel engines, a nozzle needle, which alternately closes and opens nozzle holes formed in a nozzle body, is urged in the valve closing direction by a nozzle spring. The nozzle needle has a tapered seating surface disposed in a pressure chamber formed within the nozzle body, whereby during the injection stroke pressurized fuel from an associated fuel injection pump, introduced into the pressure chamber urgingly acts upon the tapered seating surface of the nozzle needle to lift the nozzle needle against the force of the nozzle spring to effect injection of fuel through the resultantly open nozzle holes.
According to this type fuel injection valve, in a low speed and low load region of the engine the speed of fuel delivered from the fuel injection pump is so low that the injection pressure often cannot be elevated to a required level sufficient to obtain good atomization of the injected fuel. Therefore, in such low speed and low load region of the engine, the discharge area of the nozzle holes should desirably be reduced so as to achieve satisfactory atomization of the injected fuel. On the other hand, however, in a high speed and high load region of the engine, the injection quantity per unit time should be large enough to assure required high engine output, and to this end the discharge area of the nozzle holes should desirably be increased in such high speed and high load region of the engine. However, the above conventional fuel injection valve is not adapted to vary the discharge area of the nozzle holes. Therefore, if a fuel injection valve of this type is designed to have a nozzle hole discharge area appropriate to a high speed and low load region of the engine, the injection pressure can be too low to obtain satisfactory fuel atomization in a low speed and low load region of the engine.
To overcome this disadvantage, fuel injection valves employing two coaxially disposed nozzle needles have been proposed by Japanese Provisional Patent Publication (Kokai) No. 53-110722, in which a first nozzle needle is formed therein with an axial hole through which a second nozzle needle is slidably fitted. The first and second nozzle needle are urged by respective nozzle springs whose valve opening pressure is set at different values, in directions closing respective groups of nozzle holes. When the engine is operating in a low speed and low load region including the idling region, the first nozzle needle alone is lifted, whereas when the engine is operating in a high speed and high load region, also the second nozzle spring is lifted together with the first nozzle spring at the same time, to effect fuel injection through all the nozzle holes, thereby increasing the effective overall nozzle hole discharge area.
According to the proposed nozzle needle arrangement, however, it is difficult to set the urging force of the nozzle spring urging the second nozzle to a proper value with accuracy. As a result, it cannot be assured that the second nozzle needle is lifted in operating regions of the engine where the fuel injection quantity should be increased, and is kept in a seated position to close its nozzle holes in other operating regions of the engine.